1. Field of the Invention
One embodiment of the present invention relates to a light-emitting device including a tandem element.
2. Description of the Related Art
Commercialization of organic EL displays is accelerating. Displays are increasingly required to provide high luminance for outdoor use. It is known that the luminance of an organic EL element increases in proportion to electric current and light emission at high luminance can be achieved.
However, a large current flow accelerates deterioration of organic EL elements. Thus, if high luminance can be achieved with a small amount of current, light-emitting elements can have longer lifetime. In this regard, a tandem element in which a plurality of light-emitting units is stacked has been proposed as a light-emitting element capable of providing high luminance with a small amount of current (see Patent Document 1, for example).
Note that in this specification, a light-emitting unit refers to a layer or a stacked body which includes one region where electrons and holes injected from both ends are recombined.
A tandem element in which n light-emitting units of one embodiment are stacked between electrodes can provide light emission comparable to that of one light-emitting element (single element) by making current with a density of 1/n of that of the light-emitting element (single element) flow through each light-emitting unit. The tandem element can achieve n times as high luminance as the single element at the same current density.
One problem of a light-emitting panel in which tandem elements are provided adjacently is occurrence of a crosstalk phenomenon. The crosstalk phenomenon refers to a phenomenon in which, in the case where a highly conductive layer is provided in adjacent tandem elements, current leaks from one tandem element into another adjacent tandem element through the highly conductive layer.
A tandem element includes a stack of a plurality of layers with a highly conductive intermediate layer therebetween, and includes a layer with high conductivity and a layer with low conductivity because of its structure. In addition, in the tandem element, a highly conductive carrier-injection layer containing a mixed material of an organic compound and a metal oxide, a conductive high molecular compound, or the like is often used in order to decrease driving voltage. Furthermore, in the tandem element, electrical resistance between an anode and a cathode is higher than in a single element; thus, current is easily transmitted to an adjacent pixel through the highly conductive layer.
FIG. 12 is a schematic diagram illustrating occurrence of a crosstalk phenomenon due to a highly conductive intermediate layer 86. FIG. 12 shows a cross section of a light-emitting panel (white panel) including three tandem elements arranged in the form of stripes and configured to emit white light, in which only a second tandem element (B line, blue line) is driven.
The light-emitting panel includes first to third tandem elements which are adjacent to one another. The first tandem element (R line, red line) is provided between an upper electrode 81 and a first lower electrode 82. The second tandem element is provided between the upper electrode 81 and a second lower electrode 83. The third tandem element (G line, green line) is provided between the upper electrode 81 and a third lower electrode 84.
In each of the first to third tandem elements, a first light-emitting unit 85, the intermediate layer 86, and a second light-emitting unit 87 are sequentially stacked. For example, when the first light-emitting unit 85 includes a light-emitting layer capable of emitting blue light and the second light-emitting unit 87 includes a light-emitting layer capable of emitting green light and a light-emitting layer capable of emitting red light, each tandem element can provide white light emission.
In FIG. 12, a light-transmitting electrode is used as the upper electrode, and a counter glass substrate 88 is provided over the upper electrode. The counter glass substrate 88 is provided with a blue color filter, a red color filter, and a green color filter which are not illustrated. The red color filter, the blue color filter, and the green color filter overlap with the first lower electrode 82, the second lower electrode 83, and the third lower electrode 84, respectively.
When only the blue line is driven in the above-described light-emitting panel by application of a voltage between the second lower electrode 83 and the upper electrode 81, current might leak into the adjacent first or third tandem element through the highly conductive intermediate layer 86, causing the red or green line to emit light and a crosstalk phenomenon to occur.
FIG. 13 is a schematic diagram illustrating occurrence of a crosstalk phenomenon due to a highly conductive carrier-injection layer (hole-injection layer or electron-injection layer) 89, and shows a cross section of a light-emitting panel (white panel) in which only a blue line is driven.
In each of first to third tandem elements, a first light-emitting unit 85 including the highly conductive carrier-injection layer 89, an intermediate layer 86, and a second light-emitting unit 87 are sequentially stacked. As an example of the carrier-injection layer 89, a highly conductive layer containing a mixed material of an organic compound and a metal oxide, a conductive high molecular compound, or the like can be given.